During the 23-year period 1972 through 1994 quarter milk samples from 1,132,958 cows originating from 36,199 herds were examined for the presence of Listeria monocytogenes. Through the period the reference population amounted to 12,742,600 cow years and 401,682 herd years. The percentage of cows infected with L. monocytogenes varied from 0.01 to 0.1% (mean 0.04%) and of herds with an infected cow from 0.2 to 4.2% (mean 1.2%) through the period, showing a low but constant level of infection. A comparison of 33 isolates from bovine mastitis and 27 human clinical isolates was made by sero- and ribotyping. Serotyping showed that all bovine and 17 (63%) of the human isolates belonged to serogroup 1, whereas 10 (37%) of the human isolates belonged to serogroup 4. Ribotyping using EcoRI as restriction enzyme divided the 60 isolates into 16 different types, 7 of which were found among both the bovine and human types. The combination of the typing methods showed that 26 (79%) bovine and 13 (48%) human isolates shared common types. This study showed that a low but constant percentage of Danish dairy herds have cows infected with L. monocytogenes and that some of the bovine types could be found among types causing human infections.